Conventionally, strain sensors have been extensively used for structures like buildings, bridges, and tunnels, means of transportation like aircrafts, ships and trains, detecting distortions and displacement of the ground. A strain sensor provided with a resistor has been known. The strain sensor of this kind obtains strain amount on the basis of changes of electric resistance of the resistor caused by deformation. When strain is measured at many points, as an object area to be measured extends a wide area, many strain sensors are arranged on a surface of a measuring object and changes of the electric resistance of each of the sensors are measured. In this case, in each of the strain sensors wires are installed so as to measure the electric resistance and electrical power is needed to be supplied through the wire.
According to the above-described electric strain sensor, very little changes of the electric resistance must be detected and a countermeasure against electric noise is required in order to secure measurement accuracy. Further, a life of the electric strain sensor is short due to aging of the resistor, the wire and contacts thereof and there is a risk of damages by lighting.
Accordingly, a strain sensor provided with an optical fiber has been used in recent years (for example, refer to patent documents 1, 2 and the like). The strain sensor of this kind obtains strain amount on the basis of changes of reflected light and scattering light which are transmitted through the optical fiber in response to deformations of the optical fiber. Such a strain sensor does not need a supply of the electrical power and results of measurement by the strain sensor are less affected by the electric noise. Further, such a strain sensor is less aged and is less damaged by lighting since there is no need for wiring in each of the strain sensors. Therefore, it is characterized that the strain sensor has superior long-term reliability. In addition, the optical fiber itself serves as both a sensor and a signal transmission line so that signals can be transmitted over long distance. Thus, strain of an object over several kilometers can be measured by connecting a measurement instrument at one end of the optical fiber.
For example, in the patent document 1 as shown below, a strain sensor (a strain gage) measuring strain amount in an axis direction of an optical fiber linearly arranged on a carrier is disclosed.
In the patent document 2, a rosette strain sensor measuring strain amount and strain directions is disclosed. In the rosette strain sensor, single-axis strain sensors as disclosed in the patent document 1 are arranged in different directions on the same surface or on a surface parallel to each sensor. The rosette strain sensor obtains two-dimensional strain (the strain amount and the strain directions) generated on a surface of a measuring object on the basis of the strain amount measured by each strain sensor. According to the patent document 2, such rosette strain sensor is realized by an optical fiber containing fiber Bragg gratings (FBGs) which have different Bragg wavelengths and are connected in series. In this configuration, each FBG is arranged so as to direct to different directions respectively by bending the optical fiber. Then, forming narrow portions in bended portions in the optical fiber can lead to realization of reduction of reflection loss and miniaturization of the rosette strain sensor.